The present invention relates to a process for preparing polytetrafluoroethylene granular powder, more particularly to a process for preparing a polytetrafluoroethylene granular powder suitable for use in compression molding.
Polytetrafluoroethylene powder which is prepared by polymerizing tetrafluoroethylene in an aqueous medium in the substantially absence of an emulsifier is molded by compression molding which is similar to methods adopted in a field of powder metallurgy, differing from molding methods to which other thermoplastic resins are applied such as melt-molding, melt injection molding and hot compression molding. Therefore, the polytetrafluoroethylene granular powder for molding is required to have some specific properties. As the fundamental properties, it is required to have excellent flowability, to be soft and to be pressed by a comparative low pressure to form a dense preformed article, and to have a large bulk density.
As a process for preparing a polytetrafluoroethylene granular powder satisfying the above-mentioned requirements, there has hitherto been known a process wherein the polytetrafluoroethylene powder is agitated in an aqueous medium containing an organic liquid to granulate (Japanese Examined Patent Publication No. 44-22619).
Further, the above-mentioned process has been improved, and Japanese Examined Patent Publication No. 57-15128 has proposed a process for preparing a polytetrafluoroethylene granular powder having a uniformer particle size distribution and having better flowability and the process has been widely carried out. Japanese Examined Patent Publication No. 57-15128 discloses a process for agglomerating a polytetrafluoroethylene powder by agitating the same in an aqueous medium containing an organic liquid having a surface tension of not more than 35 dynes/cm wherein the agitation is carried out while the crushing of the agglomerates is conducted by using an apparatus provided with an agitating mechanism and a crushing mechanism.
According to the above-mentioned process, however, it is necessarily required that the organic liquid having the surface tension of not more than 35 dynes/cm is used in an amount of 20% to 300% by weight based on the polytetrafluoroethylene primary particles, for improving the powder flowability. Such an organic liquid is expensive, so it is desirable that the liquid is not used if possible. Also, it tends to restrict the use of widely used fluorine-containing organic liquids such as trichlorotrifluoroethane all over the world, because it is said that the fluorine-containing organic liquid destroys the ozone layer.
An object of the present invention is to provide a process for preparing a polytetrafluoroethylene granular powder which is large in bulk density, is dense and is excellent in electric properties, without using any of the organic liquids as mentioned above.
This and other objects of the present invention will become apparent from the description hereinafter.